The present invention relates to xe2x80x9cactive implantable medical devicesxe2x80x9d as such devices are defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, more particularly to pacemaker, defibrillator and/or cardiovertor devices that are able to deliver to the heart stimulation pulses of low energy for the treatment of heartbeat rate disorders.
It is known to sense cardiac activity by detecting electronic signals in the body. From among these signals, cardiac parameters, such as impedance values, electrocardiograms, and the like, can be determined and used in monitoring and treating cardiac conditions. It is known, for example, to measure an intracardiac impedance, for examples, the inter-ventricular impedance (between the right ventricle and the left ventricle) in a multisite type device, which is a useful parameter for controlling the re-synchronization of the ventricles, and the transvalvular impedance (between an atrium and ventricle on the same side of the heart) or the oblique trans-septum impedance (between right atrium and the left ventricle). These latter impedance values are correlated with the cardiac flow and make it possible, in particular, to obtain an indication of the xe2x80x9cfraction of ejection,xe2x80x9d which is a hemodynamic reference parameter for the optimization of stimulation on the various sites of a multisite device.
In the prior known systems, the cardiac parameter to be determined is sampled from a signal collected (sensed) by the implant device, typically with a sampling step of 16 ms, with the sampling and consequent measurement being repeated with each cardiac cycle. The data sampled is then used to determine the cardiac parameter in a known manner.
It is an object of the present invention to provide an improved sampling process, making it possible to determine a selected cardiac parameter while decreasing the number of collected samples required to determine the parameter, and consequently reducing the energy consumption of the implant. It is a further object to do so without deteriorating the quality of the determined result (which is related to the sampling rate, and which must be sufficiently high).
To this end, the present invention proposes to determine the selected cardiac parameter by carrying out, in a repeated manner, over a plurality of successive cardiac cycles, the detection of the moment of occurrence of a distinct cardiac event, and sampling the signal from which the aforesaid cardiac parameter will be determined during the cardiac cycles with a constant sampling step and a predetermined temporal shift between the occurrence of the detected cardiac event and the first sample of each cycle, such that the temporal shift is a variable progressive shift from one cycle to the following cycle. Preferably, the maximum duration of the variable temporal shift is always less than the duration of the current cycle, and increases by a constant increment from one cycle to the following cycle. Once the samples are obtained for a given number of successive cardiac cycles, the collected samples can be synchronized with reference to the distinct cardiac event of each cycle, and averaged or smoothed to provide a representative signal which is used to determine the aforesaid cardiac parameter. Preferably, the distinct cardiac event is one which can be used to indicate a cardiac cycle frequency, for example, a ventricular event (a spontaneous detection or a stimulation). It is possible, however, to use other events.